Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Battery-Capacitor-Solar Combination

  1. Jul 12, 2017 #1
    Hi I need some help with my electric system of my camping trailer.

    I have a 220V camping fridge freezer. On start-up it draws about 7A.
    I have a 1000W inverter connected to 2 x 105Ah 12V batteries connected in parallel.
    Connected to the batteries I have a 20A PWM solar charger / controller with 2 x 18V 120W 7.85A Solar panels connected in parallel.

    I am thinking is to run 1 x solar panel connected to the batteries to charge during the day and use only the batteries in the evening. The other solar panel connected to 6 x 3000F 2.7V Super Capacitors connected in series with balancing circuit, connected to the other solar panel and inverter for daytime use to run the fridge freezer. This prolonging my stay in remote areas without grid electricity. Will this work and what are your take on this?
    Last edited by a moderator: Jul 12, 2017
  2. jcsd
  3. Jul 12, 2017 #2
    Without being a Wizz in solar there are a couple of points which are relevant to calculate.

    How many watts does your fridge freezer use when running?

    What other drains are placed on your electrical usage, Lights, TV, etc?

    Are they using the same battery source?

    How much light will you have available for Solar charging, No. of hours day, time of year etc?

    Once you calculate your total requirements (max load) then you will know how much power your solar system needs to produce (don't forget to include any potential losses).

    Alternative and / or Emergency Back-up would be a portable Generator from Honda or similar. Is not weather dependant.

    Make sure your Starter battery is always charged up!

    Hope the comments help.
  4. Jul 12, 2017 #3
    Thank you Wizard for your response.
    My response to your questions:
    1) 4.85A x 220V = 1067W
    2) None. Lights & other utilities are using different battery bank.
    3) Two batteries dedicated to the fridge / freezer.
    4) About 6,5 hours of Summer sun light hours available per day.
    Thank you.
  5. Jul 12, 2017 #4
    Hmmn, my reckoning would say that your F/Freezer is running 24hrs, aprox 17,5 hrs on Battery power and only 6,5 on Solar then it might be better to rig the Solar to charge both batteries full time and run the F/Freezer just through the batteries.
    That way the F/Freezer will run using only 50% from each panel while the solar power is available, with the surplus going to charge the batteries at the same time otherwise using your proposed system, 1 solar panel will have to recharge 17,5 hrs of used battery power alone and the other will only be used for 6,5 hrs. to run the F/Freezer. This way you will have a total combined charging capacity of 13 daylight hours.
  6. Jul 12, 2017 #5
    Thank you for the contribution and help. I have a clear picture what to do!
  7. Jul 12, 2017 #6


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    That is not really relevant. The charge controller will be supplying as much current as it can from PV to the battery at any time during the day, which may or may not be supplying the fridge. No one would consider switching between one source and another for the fridge - in fact the fridge compressor may not even start when connected to the PVs except in full sunlight (if at all). You would need the short term high current available from the battery in order to be sure the compressor will start when the thermostat kicks in. What matters is total Amp Hours produced by PVs and total Ah used by load. The OP doesn't appear to have stated the actual number of Ah that the PVs supply each day so we can't know if the system is sustainable. The peak power / current from the PV panels is only half the story. The hours of sunshine need to be combined with where the panels are actually pointing and the irradiance.
    Last edited: Jul 12, 2017
  8. Jul 13, 2017 #7
    Your R/F uses about 17 kWhrs of electricity per day. Your panels can produce about 1.56 kWhrs per day Your batteries provide about 1.25 kWhrs per day ( you should only discharge your batteries to 50% of their capacity to optimize their useful life). With this system your are about 14kWhrs short. But that is not the first problem The R/F uses 1kW of power your panels can provide only 0.23kW so you batteries must make up the difference and provide 0.770 kW. This means the draw on the batteries must be 34 A from each. Which means that you should only use the batteries for about 1.5 hrs although it probably is less.

    Anecdotally on my boat I had a 12V DC R/F system that drew about 5+A. (70W). I had two 130W panels , 660 AHs of battery capacity and a Wind generator (about 60 W average when there was wind). I had to recharge the batteries (used only 50% of capacity) with the engine about every five days. You use about 13 times more electricity and have 3 times less battery capacity. It looks like you can only run you system about one hour after the sun sets.

    I was not overly careful with my calcs. But, with your energy/power requirements and your power sources It seems to me you will not be able to operate your system for more than a half day most likely less even draining your batteries.

    My advice is get a more efficient R/F or down size it.
  9. Jul 13, 2017 #8

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    or get a propane fridge.
  10. Jul 13, 2017 #9
    Yes. Propane is so ubiquitous and much more trouble free. low maintenance .Not too good for sailboats though, they are tippy.
  11. Jul 14, 2017 #10


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Not very Green, young man! And they are hideously inefficient, as you know.
    The verb is more like "could". There are many factors that can reduce that significantly (half or less). For a really reliable service, you have to over-kill by a wide margin - after all, you don't want the ice for your Martinis to melt and the fillet steak to go off.
  12. Jul 14, 2017 #11


    User Avatar
    Gold Member

    The OP hasn't stated the duty cycle of the fridge.
    Depending on the efficiency of insulation, how many times the door opens/closes, ambient temperature, temperature settings, the fridge could average as a rough guide low as 1/10 th of that to 1/3. Something the OP will have to figure out himself.
    At 1.7 kWhrs day the system just matches for a duty cycle of 1/10 and a sunny day.
    Any thing different from that, as you say, he is underpowered, or under-energized.
  13. Jul 14, 2017 #12

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    I was only thinking of practicality

    240 watts ?

    240 watts X 6.5 hours = 1.56 kwh a day

    i think his fridge consumption number is high, but we must add in inverter's inefficiency so i'll just use his 1067

    1.56 kwh / 1067 watts =1.43 hours per day of run time he can eke out of the sun , for the rest he'll need a generator.

    Revisit with more realistic fridge power consumption ?
  14. Jul 14, 2017 #13
    That is correct. Even in the best condition my 130 W panels actually only delivered to my battery about 105 W, On top of that you lose energy in charging the batteries and in converting DC to AC.

    The best way to assess your system is to install a battery monitor ($200) that measures the net Amp hours delivered by the batteries under actual conditions then you can determine how to augment the capacity/charging capability.
    I should not have ignored that.

    In thinking about the R/F under discussion seems like an industrial strength unit even for a household unit let alone one for an RV. How many of us have R/F that operate off a 220V line My Kenmore kitchen R/F with a rating of 120V at 3.5 amps max load and that capacity seems above what a camping trailer would require.
  15. Jul 14, 2017 #14


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    There is no 'right' solution for this problem because the reliability dimension is very relevant. If you are prepared to keep the fridge doors closed when the power fails, you can save a lot on installation cost. Buying a small petrol generator to cover extreme conditions could be an even cheaper solution than installing vast areas of PV that are seldom working at capacity.
    It All Depends . . . . . .
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted